Various types of equipment require the use of seals about axially moving parts of the equipment to prevent the flow of fluids from one or more locations into an undesired location. One instance of this is in the spray painting industry. Spray guns typically combine liquid paint and compressed air to form a stream of paint droplets directed toward a surface to be painted. Needle valves are typically utilized to control the flow of paint and air. Seals are installed about the valve stems or needles to prevent leakage of paints, air, and other fluids from the fluid conduits into the internal mechanisms of the spray gun. Leaking paint can eventually harden to prevent operation of the spray gun and can accelerate wear of gun components. These seals must be able to withstand wide ranges of pressure, and should have long service lives.
One type of commercial seal presently available is the chevron "V" packing seal. This packing material takes the shape of a chevron or "V" and can be made of any of a number of materials. Normally, this type of seal requires manual adjustment when first inserted into an annular void between the valve needle and the gun housing to pre-seat the packing. Pre-seating the packing involves the tightening of a gland nut to axially compress and radially expand the packing material, seating the packing material and initiating the seal. The gland nut is then loosened until the needle moves freely. Two obvious disadvantages with this type of seal are that pre-seating is required and that the adjustment of the packing material is dependent upon the operator's skill.
Another problem is maintaining a tight seal once the packing is installed. Existing "V" type packing seals often have some pockets or cavities spaced along the length of their inside diameters. As the packing material wears, the remaining packing material has to be further compressed by tightening the gland nut to maintain a tight seal. When the seal finally has to be replaced, the remaining packing material is tightly compressed deep within the annular void between the valve needle and the housing, making removal difficult.
Another type of seal is described in U.S. Pat. No. 4,406,468. This seal includes a packing member with a sharply cut-off tubular end. A stop member with a conical recess is positioned at an interior end of the void between the valve needle and the housing. Both the packing member and the stop member are coaxial with the valve needle. A spring is provided which forces the tubular end of the packing member into the conical recess of the stop member to form a fluid-tight seal. The sharp cut-off of the tubular end ensures that the compressive sealing forces occur at the front edge of the seal only, which reduces the amount of friction along the remainder of the packing. As the seal wears, the spring expands to force the packing member further into the conical recess to maintain a fluid tight seal. Thus, this seal is self-adjusting. The force which must be exerted by the spring in order to obtain a fluid tight seal is low, and the packing member does not need to be radially expanded. The replacement of worn seals is therefore easier than was known with existing "V" type packing seals.
Another advantage of this type of seal was it's improved service life compared to the "V" type packing seals. However, especially in the case where the fluid being sealed is a paint which includes abrasive materials, eventually abrasive particles impact themselves in the packing material and abrade grooves in the valve needle. The seal fails when a groove is formed of sufficient size to allow fluid to pass.
Additionally, it has been common practice to design the fluid (paint) seals for spray guns independently of the air seals, even when they sealed different portions of the same valve needle. Thus, air seals were provided in one portion of the spray gun, and that portion of the spray gun would have to be disassembled to service the air seals. Fluid seals were provided in a separate portion of the spray gun, which would have to be disassembled to service the fluid seals. It is often desirable to service the fluid and air seals during a single maintenance operation. In this case, two separate portions of the spray gun would have to be disassembled and reassembled.
Therefore it would be desirable to provide an improved structure for seal assemblies which lengthens the service life of the seal assembly, and which facilitates servicing of the seals.